To provide a lock nut for an inner ring for a bearing in a configuration capable of properly regulating a state where oil for lubrication is supplied to a bearing side and in a rational configuration further facilitating manufacture, and a two-shaft rotary pump using the lock nut.

For preventing axial displacement exerted to an inner ring 17a mounted to a rotary shaft (21, 22) of a bearing 17, a screw part 51 to be screwed with the rotary shaft (21, 22) is provided, and as a regulation configuration for regulating a state where oil is supplied to the bearing 17 side, a portion on a side contacting an end surface of the inner ring 17a of the bearing is formed as a tubular part 52 having an outer diameter smaller than the inner diameter of an outer ring 17b of the bearing 17. The side circumference shape of the tubular part 52 is shaped in a tapered shape to be diametrically reduced toward a tip side contacting the end surface of the inner ring 17a of the bearing, and/or irreguralities 53 for guiding a flow of oil is provided on a side circumferential surface of the tubular part 52.